The present invention relates in general to impact copolymers and more specifically to a polypropylene having a bimodal rubber particle size distribution. The polypropylene impact copolymers of the present invention achieve a desirable balance between stiffness and impact strength.
Those skilled in the art realize that rubber phase characteristics, such as rubber particle size, particle size distribution, particle morphology, rubber phase volume and degree of interfacial adhesion between rubber particles and matrix polymer significantly influence the properties of impact modified polymers. The optimization of these characteristics is critical in obtaining desirable polymer performance particularly in terms of impact strength, toughness, stiffness, and gloss.
Various efforts have been made to develop rubber-modified polymers having a balance between the properties of impact strength, stiffness and gloss since the introduction of rubber-modified thermoplastics. The concept of bimodal rubber particle size distribution was explored in styrene-containing polymers such as high impact polystyrene and acrylonitrile-butadiene-styrene (xe2x80x9cABSxe2x80x9d) in the early 1980""s. It was demonstrated that a bimodal rubber particle size distribution in high impact polystyrene provides a stiffness-ductility balance while maintaining high gloss. This is accomplished by the presence of small amounts (typically 2-10%) of large sized (5-15 xcexcm) rubber particles in small sized (0.2-1 xcexcm) rubber particles. Such materials were developed to compete with costly ABS materials.
As a general rule in rubber-modified styrene-containing polymers at a given rubber concentration, the larger the rubber particles, the higher the impact properties, but the poorer the stiffness and gloss. Those higher impact properties were attributed to the more efficient use of rubber phase. The converse has been reported for impact modified polypropylene prepared via melt blending, i.e., the smaller the rubber particles, the higher the impact strength, above 0.5 xcexcm, for a given rubber concentration. (See, B. Z. Jang et al., Poly. Eng. and Sci., 25, 643 (1985)). Because no craze formation was observed when the rubber particles were smaller than 0.5 xcexcm, the materials had lower impact properties. Crazes are a large number of tiny voids formed in an attempt to release applied energy. The differences between rubber-modified styrene-containing polymers and melt blended polypropylene may be attributed to the existence and/or the amounts of inclusions within rubber particles as well as such fracture mechanisms as crazing and shear yielding. Effects of rubber concentration and particle size on notched Izod and brittle transition temperature of rubber modified polypropylene have been reported in A. van der Wal et al, Poly. Mater. Sci. Eng., 70, 189, (1993).
Therefore, a need exists in the art for an impact polypropylene copolymer with a bimodal rubber particle size distribution which will overcome the above-listed deficiencies. The present invention provides an impact copolymer which exhibits a good balance between the properties of stiffness and impact strength.
The present invention provides a composition comprising at least about 80% of a first polypropylene having dispersed therein rubber particles of first size and a second polypropylene having dispersed therein rubber particles of a size greater than the first size.
The present invention further provides a composition comprising at least about 80% of a first polypropylene having dispersed therein rubber particles of a first size, a second polypropylene having dispersed therein rubber particles of a size greater than the first size and a third polypropylene having dispersed therein rubber particles of a size greater than the first size.
The present invention yet further provides a method of making an impact copolymer, the method comprising combining a first polypropylene having dispersed therein rubber particles of a first size and a second polypropylene having dispersed therein rubber particles of a size greater than the first size wherein the first polypropylene comprises at least about 80% of the impact copolymer.
The present invention still further provides a method of making an impact copolymer, the method comprising combining a first polypropylene having dispersed therein rubber particles of a first size with a second polypropylene having dispersed therein rubber particles of a size greater than the first size and with a third polypropylene having dispersed therein rubber particles of a size greater than the first size, wherein the first polypropylene comprises at least about 80% of the impact copolymer.
These and other advantages and benefits will be apparent from the Detailed Description of the Invention herein below.